1. Field of the Invention
The present invention relates to a bit-wise optical data storage structure and method for writing and reading.
2. Discussion of the Related Art
Capacities of commercial optical data storage systems have increased due to increasing demand. One way to increase capacity is to stack multiple layers on a single disc. Using a multiple layered-system is very attractive, because the system capacity can be increased by a factor equal to the number of layers.
Several multiple-layer data storage technologies are under investigation. For example, dual-layer recording is now at a practical engineering stage for system development in several formats, such as DVD and BD, and performance of systems using more than two conventional reflective layers for BD has been reported. Performance of systems using two-photon fluorescent media with 100's of layers is understood, but these systems exhibit low readout data rate with a single beam optical pickup. It is known that the number of layers using conventional thin-film technology is mainly limited by layer transmittance.
Conventional thin-film technology is used to optimize recording layers in single-layer and dual-layer optical discs. Extension of this technology to a many-layered disc requires special considerations. For example, each layer must reflect some amount of light in order to provide a readout signal. A deep layer suffers from decreased illumination, due to reflection losses from other layers above it. In order to write data, the layer must be exposed with at least the threshold irradiance level. Since the maximum laser power available from the source is fixed, the maximum number of layers is determined by the deepest layer that has adequate irradiance for writing. The laser power reaching the j-th layer isPj=Tj·PL  (1)where PL is total laser power incident to the disc and T is layer transmittance. In order to write data on each layer, the laser power must be greater than the threshold irradiance Ith. Thus, irradiance of the laser spot on layer j should be larger than Ith, which is mathematically expressed as
                                                        P              j                                      s              2                                ≥                      I            th                          ,                            (        2        )            where s is spot diameter,
                    s        =                              λ            NA                    .                                    (        3        )            By substituting Equation (1) into Equation (2) and using the equality, the maximum number of layers N is given as
                    N        =                                            log              ⁡                              (                                                      s                    2                                    ⁢                                                            I                      th                                        /                                          P                      L                                                                      )                                                    log              ⁢                                                          ⁢              T                                .                                    (        4        )            A plot of the maximum number of layers N versus the transmittance T of each layer is shown in FIG. 1 for the far-field Blu-ray disc (BD) optical system parameters.
FIG. 1 shows a maximum number of layers N versus layer transmittance T for conventional thin-film multi-layer system, when no absorption is assumed. The number of layers is limited by the laser power and the threshold writing irradiance. Parameters for far-filed system are for a BD system. In FIG. 1, the laser power, threshold irradiance, and wavelength are 100 mW, 5 mW/μm2, and 405 nm, respectively (PL=100 mW, Ith=5 mW/μm2, and λ=405 nm). The near-field curve is generated using NA=1.2. For example, if each layer provides 60% transmission, the total number of layers is limited to about 10. The total capacity of a 10-layer BD would be about 250 Gbyte. Ichimura et al. (“Proposal for a multilayer read-only-memory optical disk structure,” Applied Optics, 45: 1794-1803) has demonstrated an eight-layer BD-like structure by utilizing different reflectivities at each of the layers. Other considerations, like inter-layer crosstalk, which is the ratio of signal strength detected from out-of-focus layers to the signal strength detected from the in-focus (desired) layer, are also important for determining layer design of a multi-layer thin-film disc. Even if individual layers are electronically addressable, there is a residual reflection at each layer that limits the total number of layers.
To date, characteristics and limitations of extending conventional thin-film recording technology using multiple layers have not been extensively investigated and therefore are not well understood.